Second-order pretransitional effects in the high pressure phase transition of indium nitride

Indium nitride, which has the hexagonal wurtzite structure, transforms to an NaCl lattice between 12 and 16 GPa. Prior to this transition nonlinearities occur in the wurtzite phase, particularly a steep decrease of the c/a axial ratio. The equation of state and nonlinear behavior of c, a, and c/a of InN under pressure have been calculated by using the first-principles total-energy pseudopotential method. The calculations fit with experiment up to the phase transition pressure (15 GPa), which validates their accuracy. Pursuing the calculations in the wurtzite structure above the actual transition pressure the nonlinearities appear to be caused by a virtual second-order isostructural phase transition between two wurtzite structures with unequal internal positional parameters. This second-order process is interrupted by the first-order reconstructive phase transition to the high pressure structure and remains observable only as its precursor.